A form of communication in which a radio terminal directly communicates with another radio terminal without communicating through an infrastructure network such as a base station is called device-to-device (D2D) communication. The D2D communication includes at least one of Direct Communication and Direct Discovery. In some implementations, a plurality of radio terminals supporting D2D communication form a D2D communication group autonomously or under the control of a network, and perform communication with another radio terminal in the formed D2D communication group.
Proximity-based services (ProSe) specified in 3GPP Release 12 are examples of the D2D communication (see, for example, Non-patent Literature 1). ProSe direct discovery is performed through a procedure in which a radio terminal capable of performing ProSe (i.e., ProSe-enabled User Equipment (UE)) detects another ProSe-enabled UE by using only the capability of a radio communication technology (e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) technology) possessed by these two UEs. ProSe direct discovery may be performed by three or more ProSe-enabled UEs.
ProSe direct communication enables establishment of a communication path between two or more ProSe-enabled UEs existing in a direct communication range after the ProSe discovery procedure is performed. In other words, ProSe direct communication enables a ProSe-enabled UE to directly communicate with another ProSe-enabled UE, without communicating through a Public Land Mobile Network (PLMN) including a base station (eNodeB). ProSe direct communication may be performed by using a radio communication technology that is also used to access a base station (eNodeB) (i.e., E-UTRA technology) or by using a Wireless Local Area Network (WLAN) radio technology (i.e., IEEE 802.11 radio technology).
In 3GPP Release 12, a radio link between radio terminals used for direct communication or direct discovery is called a Sidelink (see, for example, Section 14 of Non-patent Literature 2). Sidelink transmission uses the Long Term Evolution (LTE) frame structure defined for an uplink and a downlink and uses a subset of uplink resources in frequency and time domains. A radio terminal (i.e., UE) performs sidelink transmission by using Single Carrier FDMA (Frequency Division Multiple Access) (SC-FDMA) similar to that for the uplink.
In 3GPP Release 12 ProSe, allocation of a radio resource for sidelink transmission to a UE is performed by a radio access network (e.g., Evolved Universal Terrestrial Radio Access Network (E-UTRAN)). A UE that has been permitted to perform sidelink transmission by a ProSe function performs ProSe direct discovery or ProSe direct communication by using a radio resource allocated by a radio access network node (e.g., eNodeB (an eNB)).
Regarding ProSe direct discovery, two resource allocation modes, i.e., autonomous resource selection and scheduled resource allocation are specified. The autonomous resource selection and the scheduled resource allocation are referred to as “sidelink discovery Type 1” and “sidelink discovery Type 2”, respectively.
In the autonomous resource selection for ProSe direct discovery (i.e., sidelink discovery Type 1), a UE that desires transmission (announcing) of a discovery signal (i.e., Physical Sidelink Shared Channel (PSDCH)) autonomously selects radio resources from a resource pool.
In the scheduled resource allocation for ProSe direct discovery (i.e., sidelink discovery Type 2), a UE requests an eNodeB to allocate resources for announcement via RRC signaling. The eNodeB allocates resources for announcement selected from a resource pool to the UE. When the scheduled resource allocation is used, the eNodeB indicates in a System Information Block (SIB 19) that it provides resources for monitoring of ProSe direct discovery but does not provide resources for announcement.
A resource pool for ProSe direct discovery is referred to as a discovery resource pool and is configured in UEs by an eNB via broadcast (SIB 19) or dedicated signaling (RRC signaling). The discovery resource pool consists of LPSDCH subframes and MPSDCH_RB frequency domain resource blocks in a discovery period. The discovery period is also referred to as a PSDCH period.
A method for designating a discovery resource pool is described with reference to FIG. 1. The discovery resource pool consists of a subframe pool and a resource block pool. To indicate the subframe pool, the eNB specifies the length (P) of a discovery period, the number (NR) of repetitions of a subframe bitmap in the discovery period, and the subframe bitmap and its length (NB).
The length (P) of the discovery period is 32, 64, 128, 256, 512, or 1024 radio frames. In 3GPP Release 12 (LTE-advanced), one radio frame has a length of 10 milliseconds and consists of 10 subframes. The length of one subframe is 1 millisecond. Therefore, the length (P) of the discovery period is 320, 640, 1280, 2560, 5120, or 10240 subframes.
The length (NB) of the subframe bitmap is 4, 8, 12, 16, 30, 40, or 42 bits. The subframe bitmap indicates that subframes corresponding to bits in each of which “0” is set are not used for the discovery and subframes corresponding to bits in each of which “1” is set can be used for the discovery.
The maximum value for the number (NR) of repetitions of the subframe bitmap in a discovery period depends on a duplex mode, i.e., frequency division duplex (FDD) or time division duplex (TDD) and, in TDD, also depends on a UL/DL configuration. Specifically, the maximum value for the number (NR) of repetitions is 5 for FDD and TDD UL/DL configuration 0, 13 for TDD UL/DL configuration 1, 25 for TDD UL/DL configuration 2, 17 for TDD UL/DL configuration 3, 25 for TDD UL/DL configuration 4, 50 for TDD UL/DL configuration 5, or 7 for TDD UL/DL configuration 6.
Therefore, the number (LPSDCH) of subframes included in the discovery resource pool corresponding to one discovery period is obtained by multiplying the number of bits in each of which a value “1” is set in the subframe bitmap by the number (NR) of repetitions. In the example shown in FIG. 1, the length (NB) of the subframe bitmap is 8 bits and the number (NR) of repetitions is 4. Further, among the 8 bits in the subframe bitmap, 4 bits are set as usable (i.e., value “1”) (hatched subframes in FIG. 1). Therefore, the number (LPSDCH) of subframes included in the discovery resource pool is 16.
Meanwhile, to indicate the resource block pool, the eNB specifies an index (S1) of a start Physical Resource Block (PRB), an index (S2) of an end PRB, and the number (M) of PRBs. The resource block pool includes M PRBs whose PRB indexes q are equal to or greater than the start index (S1) and less than S1+M (i.e., S1<=q<S1+M) and M PRBs whose PRB indexes q are greater than S2−M and equal to or less than the end index (S2) (i.e., S2−M<q<=S2) (that is, the resource block pool includes 2M PRBs in total). In other words, the eNB can designate two PRB clusters each of which includes M PRBs as the discovery resource pool.
FIG. 2 shows an example of the discovery resource pool in one discovery period. In the example shown in FIG. 2, the number (LPSDCH) of subframes included in the discovery resource pool is 16. The subframes included in the discovery resource pool (i.e., subframe pool) can be expressed as follows:
      (                  l        0        PSDCH            ,              l        1        PSDCH            ,      …      ⁢                          ,              l                              L            PSDCH                    -          1                PSDCH              )    .
Further, in the example shown in FIG. 2, the number (MRBPSDCH_RP) of resource blocks (PRBs) included in the discovery resource pool is 16. The resource blocks included in the discovery resource pool (i.e., the resource block pool) can be expressed as follows:
      (                  m        0        PSDCH            ,              m        1        PSDCH            ,      …      ⁢                          ,              m                              M            RB            PSDCH_RP                    -          1                PSDCH              )    .
In this specification, several figures similar to FIG. 2 are used to shows a discovery resource pool in one discovery period. However, in view of the above, it should be noted that a plurality of subframes included in one discovery resource pool may not be temporally contiguous. Further, resource blocks included in one discovery resource pool include two clusters.
Next, radio resource allocation for transmission of a discovery signal (i.e., PSDCH) specified in 3GPP Release 12 will be described. Details of the radio resource allocation are described in detail in Section 14.3 of Non-patent Literature 2. As already described, two different methods, i.e., sidelink discovery Type 1 and Type 2 are specified for radio resource allocation for transmission of a discovery signal (PSDCH). In the Sidelink discovery Type 1, radio resources are allocated on a non-UE specific basis. In contrast to this, in the Sidelink discovery Type 2, radio resources are allocated on a UE specific basis. Note that regarding the Type 2, though two types, i.e., Type 2A and Type 2B had been discussed, only Type 2B is specified in the current Release 12. In Type 2B, an eNB allocates radio resources for transmission of a discovery signal (PSDCH) to a UE in a semi-persistent manner. In contrast to this, in Type 2A, which is not specified in the current Release 12, an eNB allocates radio resources for transmission of a discovery signal (PSDCH) dynamically to a UE in each discovery period (i.e., PSDCH period).
The following provides a description about allocation of resources in the Sidelink discovery Type 1. In the Sidelink discovery Type 1, a UE autonomously selects a resource value nPSDCH and determines subframes and resource blocks for PSDCH transmission as follows.
The number of transmissions of a transport block on PSDCH in an i-th PSDCH period is NSLDTX=n+1 where n is given by the higher layer parameter “discoveryNumRetx”. The parameter discoveryNumRetx is configured in the UE by the eNB by using, for example, dedicated signaling (e.g., RRC Connection Reconfiguration).
The allowed resource values nPSDCH from which the UE can select are integers from 0 to (Nt*Nf−1), where Nt and Nf are defined as follows:Nt=└LPSDCH/NSLDTX┘,Nf=└MRBPSDCH_RP/2┘.
The j-th transmission for a discovery signal (i.e., the transport block on the PSDCH) in the discovery period occurs in the subframelNSLD·b1+j−1TXPSDCH among LPSDCH subframes (i.e., l0PSDCH, l1PSDCH, . . . , lL_PSDCH-1PSDCH) in the discovery resource pool and uses two contiguous resource blocksm2·ajPSDCH and m2·aj+1PSDCH of the above-shown subframe, whereaj=((j−1)·└Nf/NSLDTX┘+└nPSDCH/Nt┘)mod Nf,b1=nPSDCH mod Nt.
FIG. 3 shows an example of radio resource allocation based on the Sidelink discovery Type 1 when LPSDCH=16, MRBPSDCH_RP=16, and NSLDTX=4. A numerical value in each cell shown in FIG. 3 indicates a value of the resource value nPSDCH that a UE can select. For example, when nPSDCH=0, PSDCH transmission is performed four times in the first, second, third, and fourth subframes l0PSDCH, l1PSDCH, l2PSDCH, and l3PSDCH in the discovery resource pool. Similarly to this, when nPSDCH=4, PSDCH transmission is also performed four times in the first, second, third, and fourth subframes l0PSDCH, l1PSDCH, l2PSDCH, and l3PSDCH in the discovery resource pool.